The Plugin should not export obviously invalid geometries

[makeclean]

A user recently had a bit of a head-scratcher where during the building of the implicit complement there was a failure, it turns out this is due to the creation of a sheet body in the model creation process.

We should do something sensible when exporting DAGMC geometries which contain sheet bodies, the obvious thing to do would be to fail and prompt the user to remove their sheet bodies.